1. Technical Field
This invention relates generally to the transmission of images with two channels, and particularly of images to be multitoned.
2. Background Art
The Joint Photographic Expert Group (JPEG) data compression standard, as described by William B. Pennebaker and Joan L. Mitchell in xe2x80x9cJPEG Still Image Compression Standardsxe2x80x9d, Van Nostrand Reinhold of NY, 1993, is a commonly used transform-based process of image compression which is easily tailored to the frequency-domain response of the human visual system.
U.S. Pat. No. 5,289,548, which issued to D. L. Wilson et al on Feb. 22, 1994, has shown that the low-frequency portion of an image may be differential pulse code modulated (DPCM) as described by Anil K. Jain in xe2x80x9cFundamentals of Image Processingxe2x80x9d, Prentice Hall of Englewood Cliffs, N.J., 1989, and then transmitted; and that JPEG may be used to transmit the remaining high-frequency information. This was done to minimize the occurrence of blocking artifacts in radiological images.
U.S. Pat. No. 5,432,870, which issued to E. L. Schwartz on Jul. 11, 1995, has described another process for transmitting an image with two channels, whereby a binary version of the image is transmitted using compression such as a Joint Binary Industry Group (JBIG) standard described by Pennebaker and Mitchell, and a visually masked residual image is transmitted using JPEG. Again, the JPEG channel is used primarily to transmit only high frequency information.
In the case of an image library, it may be desired to browse images at lower resolutions on a soft display (such as a CRT), because a soft display typically cannot display high frequencies anyway. By truncating the higher frequency components of an image within JPEG, the compression performance of JPEG is greatly increased. When a hard copy is desired, such a compressed image could then be printed, but with two deleterious effects. First, because typical hardcopy devices have good high frequency response, the artifacts caused by the prior truncation of high frequencies will be reproduced and thereby become evident. Second, many hardcopy devices require the use of multitoning (such as halftoning), as described by Robert Ulichney in xe2x80x9cDigital Halftoningxe2x80x9d, The MIT Press of Cambridge, Mass., 1987, and as illustrated in FIG. 1 hereof, to produce the illusion of a continuous tone image; and this multitoning may interact with the above-mentioned artifacts, making them even more severe.
The JPEG standard supports a xe2x80x9cprogressivexe2x80x9d mode, whereby a low bit-rate image is encoded by truncating the higher frequency JPEG coefficients. When higher quality is desired, these missing coefficients are then transmitted. This may be done multiple times, until the desired image quality is obtained; but this is a costly process because each JPEG decompression requires significant computation. A complete JPEG decompression must be done each time additional coefficients are transmitted. Thus, if an image were to be transmitted at a low bit-rate for image browsing, and then the high-frequency coefficients were transmitted, multiple JPEG decompressions would need to be done. Although such JPEG decompressions could be optimized for particular groupings of coefficients, this would lack generality.
If an image is compressed by JPEG compression such that its high-frequency components are suppressed, excessive blurring of edges would result in the reconstruction. Further, if the image is to be multitoned after reconstruction, the multitoning will inject large amounts of high frequency noise into the image. These effects will result in indistinct edges (which is undesired) and in a general presence of high frequency noise (which is not too objectionable) throughout the image.
It is an object of the present invention to provide image reconstruction by sending low frequency image information via a first transmission channel and high frequency information via a second channel.
It is another object of the present invention to provide image reconstruction by sending low frequency image information via a first transmission channel and sending the difference between (1) a multitoned original image information and (2) a multitoned low frequency image information via a second channel.
It is still another object of the present invention to provide image reconstruction by sending low frequency image information via a first transmission channel and sending high frequency edge information via a second channel. This provides the needed high frequency information which exists in the vicinity of edges, but not the high frequency information which exists in other areas of the image. Any high frequency information existing in the other regions of the image would be hidden by the multitoning noise, and is therefore not needed to provide a pleasing rendition of the image.
According to a feature of the present invention, a process for the compression and transmission of original images containing both low frequency information and high frequency information includes the steps of compressing the low frequency information; transmitting the compressed low frequency information; multitoning the low frequency information to form a multitoned reconstruction; multitoning the original image to form a multitoned original; calculating a residual signal as the difference between the multitoned original and the multitoned reconstruction; and transmitting the residual signal.
According to a preferred embodiment of the present invention, the step of multitoning the low frequency information to form a multitoned reconstruction includes the step of decompressing the compressed low frequency information. The low frequency information is compressed by lossless or lossy techniques, and the original image and the reconstruction of the compressed low frequency information are multitoned by the same method. The process may further include the step of spatially filtering the residual signal to substantially eliminate isolated non-zero pixels, thereby reducing entropy of the residual signal, as described by Majid Rabbani and Paul W. Jones in xe2x80x9cDigital Image Compression Techniquesxe2x80x9d, SPIE Optical Engineering Press of Bellingham, Wash., 1991.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiments presented below.